European farmers may have to retreat from vast areas of the countryside if new water quality targets are to be met.

Researchers addressing the British Association's science festival in Leicester said a new directive from Brussels could lead to radical changes in the way agricultural land was managed across the continent - with some types of farming being scaled back or even abandoned in some of their traditional areas.

The Water Framework Directive will require all rivers, lakes and canals to be returned to "good ecological quality" within 15 years - and the measure of quality will be far tougher than it is now.

Scientists at the festival said that to comply with this new regime, pollution - such as from the leaching of fertilisers from fields - would have to be more tightly controlled than it is now.

Land use changes

"Farmers are going to have to retreat," Dr Simon Harrison, of Ireland's University College, Cork, said. "If you want to have nice rivers there are going to have to be some changes in agricultural practices." He added: "This is a Europe-wide problem."

There are likely to have to be substantial reductions in the number of sheep or cattle that can be kept in some areas and further restrictions on the size and position of crop fields.

"In the last decade of the 20th Century, 95% of British freshwater was polluted by nutrients - most of which was from diffuse sources - in other words farming," Dr Penny Johnes, from the University of Reading, said.

"British waters are highly degraded in terms of nutrient pollution. We need to change land use."

Animal waste

She told the BBC: "We know that certain types of land are much more susceptible to losing nutrients, sediments and pesticides.

"We need to put high-risk land uses in the areas that have the best capability to hang on to nutrients and that may mean landscape-scale changes in the way we farm."

She cited the example of hillside sheep farmers and dairy herdsmen on wetlands who may be asked to reduce their livestock densities because the local weather and land conditions made it more likely that the waste from these animals would contaminate watercourses.

"The stocking densities in some places are way too high for the sensitivity of the landscape. Steep hill-slopes and moorland-fenland areas are very susceptible to losing nutrients."

Sustainable economies

Only radical proposals would tackle the nitrogen and phosphorous build-up now blighting waterways, she said.

"But all of this has to be done sensitively," she added.

"Remember that in the UK, for example, this is an industry that has been hit very hard by foot-and-mouth, BSE and all the issues the Countryside Alliance is now campaigning on.

"We can't say to farmers 'you simply can't farm here anymore'. We have to talk about alternative economies in the rural environment that are sustainable; and the government has to do this and it has to do it intensively."